This invention relates generally to the measurement of charge on ions and particles, and more specifically to the measurement of such charge using an array detector.
In certain devices and as part of some measurement techniques it is desirable to quantify, with high precision and sensitivity, the charge on individual gas phase ions (including molecular ions and charged particles and aerosols). This charge may range in size from a few tens of electrons to tens of thousands of electrons or higher, and may be created on the particle by a number of means both by naturally occurring processes or by deliberate ionization methods, including electrospray ionization, corona discharge, UV light irradiation and others. These ions, whose primary characteristic is that of being multiply charged, i.e., carrying more than a single unit charge, are generally quite massive having molecular weights that are measured in hundreds of thousands of atomic mass units at the low end on up to pico-grams at the high end. As consequence these gas phase ions travel with low or near thermal velocity even when they possess substantial amount of kinetic energy such as kilo-electron volts per unit charge. Low velocity ions will not create a signal in detectors that rely on the generation of secondary electrons or ions from a surface, nor on the generation of charge/hole pairs within a semiconductor, such as charge coupled devices (CCD""s). These ions will induce a signal in detectors that can respond to the presence of the electrostatic charge carried by an ion. What""s more the magnitude of induced signal is generally proportional to the amount of charge detected.
A practical detector for measuring charge on single ions and particles should have a number of useful characteristics. The detector should be sensitive enough to detect and quantify the charge on individual ions with charge as little as less than 100 electrons and have a noise level of not more than a few tens of unit charges. It should respond to low energy ions that come into contact with the detector surface. The detector should also possess a usable working area to which ions may be delivered in order to maintain a reasonable detection efficiency. It should not require vacuum for operation but should be able to operate in vacuum or any ambient gas pressure. Ideally the detector will not require cooling for low noise operation. It should also be compact, robust, and inexpensively fabricated with microelectronic technology. As such the detector CCD detectors have many of the characteristics mentioned above and they detect charge with high sensitivity, but that charge must be created and collected inside the silicon of the device by some energetic process before it can be transferred to an amplifier. The presence of an electrostatic charge is not sufficient to create the necessary physical charge inside the device. CCD""s therefore cannot be used to directly sense charge deposited on the surface of their pixels.
The present invention provides a tessellated array detector with charge collecting plate (or cup) electrode pixels and amplifying circuitry integrated into each pixel making it sensitive to external electrostatic charge; a micro collector/amplifier pixel design possessing a small capacitance to ensure a high charge to voltage signal conversion for low noise/high sensitivity operation; a micro-fabricated array of such pixels to create a useful macroscopic target area for ion and charged particle collection.